1. Field of the Invention
The invention relates to a furnace for heating a preform, especially a plastic container preform.
The invention relates more particularly to a furnace for heating a preform, especially a plastic container preform, which comprises a longitudinal heating tunnel bounded transversely by the mutually parallel longitudinal vertical internal faces of two walls, one a heating wall, being equipped with a heating system, and the other an aerated wall, having aeration orifices that are intended to let the air blown by a blower pass through them transversely from the upstream, outside the heating tunnel, to the downstream, inside the latter, and in which furnace a first portion of the preform is heated in the heating tunnel, while the second portion of the preform is held outside the heating tunnel through a longitudinal opening made between the longitudinal edges of the walls of the heating tunnel, the blower blowing air transversely to the second portion of the preform so as to keep the constituent material of this second portion of the preform at a temperature below its softening point.
2. Description of the Related Art
Plastic containers, and especially bottles, for example made of polyethylene terephthalate (PET), are generally produced from preforms that are shaped into bottles by an operation of blow or stretch-blow moulding of their body and their end wall. The preforms generally have the form of a test tube, the neck of which already has the final shape of the throat of the container to be produced. The neck has for example a thread intended to receive a screw stopper.
Before carrying out the blow moulding operation, it is necessary to heat the preforms in a heating furnace so as to soften the plastic. For this purpose, the constituent plastic of the preforms is heated to a temperature above its glass transition temperature.
Preform heating furnaces of the type having a longitudinal heating tunnel are already known. The cold preform is transported by a transport device from a first end of the tunnel and then travels along the tunnel, in which it is heated, before emerging via the second end of the tunnel. The preform thus heated is ready for the blow moulding operation.
To ensure depthwise heating of the end wall and the cylindrical wall of the body of the preform over its entire periphery, the preforms are generally rotated about themselves as they travel through the furnace. Moreover, one wall of the tunnel is equipped with radiation heating means, while the other wall is provided with aeration orifices for blown air to flow through so as to promote uniform heating throughout the thickness of the cylindrical wall of the body of the preform without overheating the layer of surface material forming the external face of the cylindrical wall of the preform.
In fact, the blown air removes the convective heat caused by the heating means, in order to promote penetration of the radiation that it produces in the thickness of the constituent material of the body.
However, the neck of the preform, which has its final shape and its final dimensions, must not be deformed during the heating and/or blow moulding operations. It is therefore essential to keep the neck at a temperature below the glass transition temperature. For this purpose, the neck of the preform is held outside the tunnel through an opening in the tunnel that forms a longitudinal slot.
To prevent the heat from the heating tunnel from being communicated to the neck of the preform, said neck is cooled by air blown by a blower.
It is known to equip furnaces with a single blower, which makes it possible, on the one hand, to distribute the heat uniformly in the heating tunnel and, on the other hand, to cool the neck of the preform. The blower is then equipped with a deflector, which is located upstream of the tunnel and of the preform relative to the flow direction of the air. The deflector thus deflects a portion of the stream of blown air on to the neck of the preform.